This invention relates generally to energetic particle detectors and is particularly directed to a coated semiconductor detector capable of detecting cold, thermal and epithermal neutrons.
Semiconductor detectors coated with neutron reactive materials offer an alternative approach to scintillator-based neutron imaging devices for neutron radiography (normally scintillating screens coupled to photographic film or to other photorecording devices). Neutron reaction film-coated devices typically include Si, bulk GaAs, or diamond detectors, all of which have advantages and disadvantages. Si and bulk GaAs-based devices operate at moderately low voltages, whereas diamond-based films require hundreds of volts to operate. Although diamond-based films appear to be more radiation hard than GaAs, GaAs devices demonstrate superior radiation hardness to neutrons and gamma rays in comparison to Si. Neutron reactive films based on the 157Gd(n,xcex3)158Gd reaction show a higher neutron absorption efficiency than 10B(n,xcex1)7Li and 6Li(n,xcex1)3H-based films, however, the combined emission of low energy gamma rays and conversion electrons from 157Gd(n,xcex3)158Gd reactions make neutron-induced events difficult to discriminate from background gamma-ray events. The particle energies emitted from the 6Li(n,xcex1)3H reaction are greater than those emitted from the 10B(n,xcex1)7Li reaction and are much greater than observed from the 157Gd(n,xcex3)158Gd reaction. Yet, the optimized film thickness for 6LiF is over ten times greater than needed for 10B while producing only a slight increase in neutron detection efficiency. Background gamma rays are less likely to interact in a diamond or Si detector than in GaAs, but previous results have shown that the gamma-ray background interference for 10B-coated GaAs detectors is low enough to discriminate between neutron and gamma-ray events. Regardless, Si, GaAs, diamond, and a variety of other semiconducting materials can be used as the detector in the present invention.
Referring to FIG. 1, there is shown a simplified schematic illustration of the basic components comprising a 10B-coated semiconductor neutron detector 10. The neutron detector 10 includes a semiconductor substrate 12 having on one surface thereof a back contact layer 14 which is coupled to a potential, depicted as neutral ground in the figure. Disposed upon the substrate 12 is a front contact 18 that forms a blocking contact upon the semiconductor substrate 12. An active region 16 forms from either the blocking contact 18 potential or through the application of voltage 30 (shown in dotted line form in the figure), or a combination of both the blocking contact 18 and the applied voltage 30. An energetic neutron 22 interacts with the 10B film 20, thereby releasing an alpha particle 24 and a 7Li ion 26 in opposite directions. Only one particle from this interaction can enter the active region 16 of the semiconductor substrate 12 which limits detector efficiency. The active region 16 has an internal electric field that causes free charges 34 to separate and drift across the active region 16. The motion of the free charges 34 induces a signal to appear on preamplifier circuit 28 or other sensitive electronics. The preamplifier 28 may be connected to the voltage 30 and the detector 10 through a coupling capacitor 32. The present invention is intended to increase neutron detector efficiency, or sensitivity.
Now referring to FIG. 2, there is shown a simplified schematic illustration of the basic composition and configuration of a 10B-coated self-biased high-purity epitaxial GaAs neutron detector 11. The neutron detector 11 includes an n-type GaAs substrate 12 having on one surface thereof a back contact layer 14 which is coupled to neutral ground. The GaAs substrate 12 includes a high-purity v-type GaAs active region 16. Disposed on a surface of the high purity v-type GaAs active region 16 is a front contact layer 18 forming a small p+ GaAs layer 36. Disposed upon the p+ GaAs layer is a conductive contact 18. The p+ GaAs layer may instead be replaced by a Schottky barrier contact. A built-in potential at the of the p-type/v-type GaAs junction forms an active region 16 that supplies enough voltage to operate the device. A potential source 30 (shown in dotted line form in the figure) may also be used to power the neutron detector 11 which typically provides a detection signal to a preamplifier 28. Disposed on the front contact layer 18 is a thin layer of Boron-10 film 20. An energetic neutron 22 interacts with the 10B film 20, thereby releasing an alpha particle 24 and a 7Li ion 26 in opposite directions. Only one particle from this interaction can enter the high purity v-type GaAs active region 16 of the GaAs substrate 12 which limits detector efficiency. The present invention is intended to increase neutron detector efficiency, or sensitivity.
Accordingly, it is an object of the present invention to provide a neutron detector having increased efficiency for detecting cold, thermal or epithermal neutrons.
It is another object of the present invention to provide a highly sensitive energetic neutron detector which is inexpensive to fabricate, radiation hard, relatively insensitive to gamma-ray background radiation and is readily adapted for use in neutron radiography, imaging devices, or neutron imaging applications in other harsh radiation environments.
It is yet another object of the present invention to provide an energetic neutron detection device which can be operated at room temperature at low voltages and which is compact and rugged.
The present invention contemplates apparatus for detecting energetic neutrons comprising: a particle detecting semiconductor substrate having a first surface including at least one cavity extending into said semiconductor substrate; and a thin neutron responsive layer disposed on the first surface of the semiconductor substrate and responsive to energetic neutrons incident thereon for producing first and second charged reaction particles directed in opposite directions, wherein the neutron responsive layer is further disposed in said at least one cavity for increasing neutron detection efficiency by increasing the likelihood that the charged reaction particles will be directed into the semiconductor substrate.